The goals of this project are to 1) continue to purify and immunologically characterize the structural and functional proteins of mammalian retroviruses, 2) utilize radioimmunoassays for these polypeptides to search for retrovirus expression in cells and tumors with special emphasis on those of human origin, 3) isolate recombinants between mouse type-C viruses by both in vivo and in vitro tissue culture techniques, 4) study the biological properties of such recombinant viruses in order to find genetic markers for oncogenicity of murine leukemia viruses, and 5) establish the genetic map of such recombinants by immunologic and biochemical techniques.